Nonplanar Monocyanines: Meso-Substituted Thiazole Orange with High Photostability and Their Synthetic Strategy as well as a Cell Association Study

A convenient approach for the direct synthesis of meso-substituted thiazole orange (meso-TO) analogues has been unprecedentedly developed through the AlCl3-catalyzed reaction of parent TO with benzyl alcohol derivatives. Single-crystal X-ray structures show that the prepared new meso-TO analogues are nonplanar, forming a sharp contrast to planar TO. The spectral properties show that nonplanar meso-TO analogues do not aggregate, existing in monomer form (M) in PBS buffer, and have little effect of solvatochromism in different solvents. In comparison with the parents, meso-TO analogues exhibit a large Stokes shift, excellent light fastness, and inertness to singlet oxygen. A cellular association study demonstrates that incorporation of a benzyl group at the meso position methine of parent TO can decrease the cytotoxicity, change staining area in cells, and emit long-wavelength fluorescence for an extended time, which are useful for the development of smarter TOs for imaging in biological science.